With the advent of digital multimedia data and digital multimedia data distribution, protection of such digital multimedia data against unauthorised copying and dissemination have become an issue for multimedia data publishers and authors. One technique used to identify the ownership of an image is to embed a pattern or patterns into the image, such that the embedded pattern is not visible to the naked eye of an observer. Such a pattern is called a watermark. The presence of the watermark can be detected in a copied image by the owner of the original image, thereby proving their ownership.
Systems are known for embedding a pattern or patterns into an image. Several watermarking schemes have also been developed to imperceptibly embed information in an image that may later be retrieved. This embedded information is commonly used to check the provenance of the image, record information about the image that is not directly visible, or even store information totally unrelated to the image.
Such information will remain with the image even if the header and other metadata of the image file are removed. The information may be usefully employed to define the address or location of original metadata related to that image. The metadata may contain owner identification, camera settings, geographical location, details of the subjects in the image, or any number of pieces of other information. The image metadata to which the information provides the address or location of may be stored on the same device as the image, or on a server connected by a network, or even a server owned by a third party on the Internet.
The information stored in a watermark of an image may also be used for fingerprinting, which is used to trace the source of illegal copies. In this case, the owner can embed different information in the copies of the image supplied to different customers. By comparing the information extracted from illegal copies to that added to the copies of the image supplied to customers, the customers who have broken their license agreement by supplying the data to third parties may be identified.
Another use of such information stored in a watermark of an image is to directly control digital recording and playback devices for copy protection purposes. In this case, the watermark can include copy- and playback-prohibit information. A watermark detector in the recording and playback device may use the prohibit information to prohibit copying or playback of such an image.
Yet another reason for using a watermark to store information in an image is simply to hide that information. As watermarks in images are typically imperceptible to the human eye, the presence of such information will be unnoticed by an observer.
A difficulty arises with respect to reliably embedding watermarking patterns in such a way that they are both imperceptible to observers of the image, and also carry enough information to be useful in their intended application area. In general, as more information is embedded in a watermark, the perceptibility of the watermark increases.